This is a request to continue follow-up from 1995-1999 of homosexual/bisexual men originally recruited into the UCLA Center of the Multicenter AIDS Cohort Study (MACS) in 1984-1985. The study cohort will include: 1) 434 HIV-1 antibody-positive men who have survived and continued to participate in the study; 2) 82 high-risk HIV-1 antibody- negative men, including a subset of men currently engaging in activities that are predictive of seroconversion and a subset of men who remain seronegative despite having had many different sexual partners; and 3) a random sample of 35 HIV-1 antibody-negative men for use as a control standard. Every six months the standard MACS questionnaire, neuropsychologic tests, and a short physical examination will be administered to the men and approximately 150 cc of blood will be drawn. The blood will be processed for serum, cells, and plasma. Equal aliquots of serum, cells, and plasma will be saved for the UCLA and NIH repositories. Serologic testing for HIV-1 antibodies will be performed on serum from HIV-1 antibody-negative men and levels of T-cell subsets determined on all the men. The men will be followed for clinical outcomes and treatment. Biopsy material and medical records will be obtained from treating physicians and hospitals, and autopsies will be performed on all men from whom we have obtained permission. The UCLA principal investigator will serve on the Executive Committee for the MACS and the appropriate local investigators will represent the UCLA Center on the various MACS-wide working groups. The UCLA Center will continue to collaborate actively with the other three clinical centers of the MACS and with CAMACS, the data management and coordinating center.